Double-sided electroluminescent device

ABSTRACT

An electroluminescent device including a layer of electroluminescent organic semiconductor material between a first transparent electrode of an n-type semiconductor material selected from nitrides and inorganic oxides, and a second electrode.

The present invention relates to an electroluminescent device comprisinga layer of electroluminescent semiconductive organic material arrangedbetween a first electrode, constituted by a material having the propertyof injecting electrons into the said layer of electroluminescentmaterial, and a second electrode constituted by a material having theproperty of injecting holes into this layer.

BACKGROUND OF THE INVENTION

Devices of this kind are already known, in which the semiconductiveorganic material consists either of a monomer organic substance,constituted by fluorescent molecules, such as anthracene, perylene, andcoronene, or by molecules of an organic dye, or of a conjugated organicpolymer such as poly-(p-phenylene-vinylene).

In these devices, the electrode which emits electrons is, for example,constituted by a layer of a metal chosen from among aluminium,magnesium, and calcium, or by a layer of metallic alloy, such as analloy of magnesium and silver, and the electrode which emits holes isconstituted, for example, by a layer of a metal such as gold or by alayer of tin oxide (SnO₂) or mixed indium and tin oxide (ITO).

Such a device is described in the international patent applicationpublished under number WO 90/13148.

Such electroluminescent devices can be used in particular aslight-emitting diodes in display elements, as well as for themanufacture of flat screens for portable computers or television sets.

These devices feature the advantage of easily allowing for themanufacture of large display surfaces, as well as allowing for anadjustment of the emitted light wavelength, therefore the emissioncolour, by selecting in an appropriate manner the semiconductive organicmaterial which constitutes the electroluminescent layer from among thelarge number of known materials, which are suitable for this purpose, aswell as the multiple combinations or modifications of these materialswhich are available to specialists.

In addition to this, these devices, in general, have a light emissionefficiency which is quite acceptable, within the current state of theart, and which seems to be susceptible to improvements in the futurewithin the capability of persons skilled in the art.

According to the prior state of the art relating to devices of thiskind, the hole-injecting electrode has been provided in the form of atransparent layer, constituted, for example, by a mixed indium and tinoxide, the electron-injecting electrode itself being opaque orreflective. Devices of this type can emit light on only one face.According to one of the variants of the device described in applicationWO 90/13148, it is however mentioned that at least one of thecharge-injecting contact layers, if these layers are of gold oraluminium and do not exceed a certain thickness, is transparent orsemitransparent. It is not however specified which of these layers istransparent or semitransparent.

In addition to this, the devices currently known feature thedisadvantage that they have a too short lifetime in regard of theenvisaged industrial applications. More specifically, the best knowndevices of this type, in which the electroluminescent organic materiallayer is constituted by a monomer organic substance, only allow for amaximum period of use of the order of a thousand hours, in continuousoperation, while the best known devices, in which the electroluminescentorganic material layer consists of a conjugated polymer, do not ingeneral resist a period of continuous operation greater than about ahundred hours.

SUMMARY OF THE INVENTION

The aim of the invention is to provide a device of the above-mentionedkind which is capable of emitting light on both its faces, i.e. anelectroluminescent device in which both the electrodes located on eitherside of the electroluminescent material layer are transparent ortranslucent.

A further aim of the invention is to allow for the improvement of thedevice lifetime.

To this end, the device according to the invention is characterised inthat the said first electrode is in the form of a transparent ortranslucent layer of a type n semiconductor material, chosen from amongthe mineral oxides and nitrides.

Favourably, the material constituting the electron-emitting electrode ischosen from among gallium nitride GaN, binary alloys of gallium nitrideand indium nitride, of the general formula Ga_(x)In_((1−x))N, ternaryalloys of gallium nitride, indium nitride, and aluminium nitride, of thegeneral formula Ga_(x)Al_(y)In_((1−x−y))N, and mixtures of at least twoof these compounds and alloys, where x and y each represent a numberbetween 0 and 1, the total of x+y being at the most equal to 1, then-conductivity characteristic of the said material resulting possiblyfrom stoichiometric defects or from doping by at least one elementchosen from among groups 4a and 6a of the periodic classification table.

As the doping element, use may be made in particular of one of thefollowing elements: Si, Sn, S, Se, and Te.

The above-mentioned type n semiconductor material, in particular galliumnitride and its alloys, may be used in any appropriate form, inparticular in monocrystalline, polycrystalline, nanocrystalline, oramorphous form, or even in the form of a superimposition of layers ofthis type having different compositions, therefore different values of xor y, or different type of doping.

Use may also be made, as the material constituting the electron-emittingelectrode, of a material chosen from among titanium oxides TiO_(x),whatever their oxygen stoichiometry may be and particularly in thesub-stoichiometric anatase and rutile phases TiO_(2−y), as well asmixtures of at least one titanium oxide with at least one other mineraloxide, particularly the multiphase materials such as the Maneli phasesor the multiphase mixtures of several oxides accompanying titaniumoxide.

The electron-injecting character of such materials may possibly resultfrom the existence of stoichiometric defects or from doping by at leastone element such as, for example, H, Li, Ca, Al, Cs.

The above-mentioned titanium oxides may be used in any appropriate form,and in particular in monocrystalline, polycrystalline, nanocrystalline,or amorphous form.

As the electroluminescent semiconductive organic material constitutingthe electroluminescent layer, use may be made of any appropriatematerial, in particular those constituted by the substances already usedfor this purpose in accordance with the prior art, in particular,conjugated polymers, such as poly(p-phenylene-vinylene), commonlydesignated by the abbreviation PPV, or poly p-phenylene, PPP, orpolythiophene, PT, those in which the phenyl or thiophene rings carrieone or more substitutents such as an alkyl group, an alkoxy group, ahalogen, or a nitro group, as well as conjugated polymers such aspoly(4,4′-diphenylene-diphenylvinylene), commonly designated by theabbreviation PDPV; poly (1,4-phenylene-1-phenylinyene);poly(1,4-phenylene-diphenyvinylene); polymers of the typepoly(3-alkylthiophene) or poly(3-alkylpyrrole), polymers of the typepoly(2,5-dialkoxy-p-phenylenevinylene), or copolymers or mixtures ofsuch conjugated polymers.

The use of conjugated polymers deriving from known polymers, such asthose mentioned above, by grafting onto the polymer chain ends groupshaving the property of strengthening the adherence of theelectroluminescent conjugated polymer onto the surface of theelectrodes, in particular the electron-emitting electrode, and, moreparticularly, onto a layer of gallium nitride or titanium oxide, isparticularly advantageous.

For example, use may be made of polymers deriving from poly(phenylene)of which the chain ends have one of the following formulae:

Use may equally be made, as the electroluminescent organic materialconstituting the electroluminescent layer, of a monomer substance, of anorganic pigment or dye, this substance or this pigment or dye beingpossibly chosen in particular from among those appropriate for use inthe electroluminescent devices of the prior art. These dyes may also bechemically modified in such a way as to adhere better to the electrodeof the invention.

As the material constituting the hole-emitting electrode, use may bemade of the same materials as those used in the electroluminescentdevices according to the prior art, in particular gold, tin oxide SnO₂,or mixed indium and tin oxide (in particular the commercially availableproduct known by the designation of ITO), in the form of a transparentlayer.

It is also possible to insert, between the electron-emitting electrodeand the electroluminescent semiconductive organic layer, one or morelayers of material which facilitates the transport of negative charges,this material consisting, for example, of compound 8-hydroxyquinolinealuminium (usually designated by the term Alq3), as well as one or morelayers of material having the property of blocking the passage ofpositive charges (hole-stopping layer), such a material being, forexample, constituted by compound2-(4-bi-phenyl-5-tertbutyl-phenyl)-1,3,5-oxadiazole (a compound knownunder the designation “Butyl-PBD”).

In addition to this, it is also possible, if appropriate, to insertbetween the hole-emitting electrode and the electroluminescentsemiconductive organic layer, one or more layers of material whichfacilitates the transport of positive charges. Such a material may beconstituted, for example, by a compound of the typediphenyl-dimethylphenylamine, known by the designation TPD.

For the manufacture of the electroluminescent device according to theinvention, use may be made of any appropriate process, in particular ofthe techniques used for the manufacture of devices according to theprior art.

Accordingly, in order to form the nitride layer as defined above, inparticular of gallium nitride, constituting the electron-emittingelectrode, use may be made of the inherently-known thermal pulverisationcoating methods, in particular by means of a plasma torch, or of thecoating techniques based on the liquid phase, as well as the coatingprocesses involving chemical reactions in the vapour phase. These latterprocesses seem to give the best results.

More particularly, use may be made favourably, in order to form a thingallium nitride layer, of a coating process involving a chemicalreaction in the vapour phase under operational conditions identical orsimilar to those described by M. Ilegems in the publication Journal ofCrystal Growth, 13/14, p. 360 (1972).

Preferably, the type n semiconductive mineral compound layer,constituting the electron-emitting electrode, is formed first on thesurface of the substrate serving as a support to the electroluminescentdevice, this substrate being constituted favourably by a transparentinsulating material such as a small plate of sapphire or quartz.

It is however likewise possible to form the layer of materialconstituting the holes-emitting electrode first on the substrate.

In order to form the titanium oxide layer as defined above, use may bemade of the inherently-known titanium oxidation methods, of sol—gelpolymerisation methods starting from organic precursors, ofpulverisation methods with the aid of plasma or ion bombardment. Theselatter methods seem particularly well-indicated.

In order to form the electroluminescent semiconductive organic materiallayer, use may likewise be made of any appropriate technique,particularly thermal evaporation processes, immersion in a solution (theso-called “dip-coating” processes), coating of a layer of substance,such as a solution of the electroluminescent material, or of precursoragents thereof, in an appropriate solvent, onto the surface of theelectron-emitting electrode (or, as applicable, of the hole-emittingelectrode), by turning the substrate (the so-called “spin-coating”process) in such a way as to obtain a perfectly uniform thickness ofthis layer, followed possibly by a thermal or chemical treatmentallowing for the formation of the film of electroluminescent materialproper.

In order to form the layer of material constituting the hole-emittingelectrode, such as gold, tin oxide, and mixed indium and tin oxide, itis likewise possible to proceed in an inherently-known manner, forexample, by evaporation under reduced pressure or by thermalpulverisation, or evaporation under vacum by bombardment by means ofelectron, ion beam, etc.

Favourably, use may be made, as the substrate, of a transparent ortranslucent material, and the thicknesses of the layers of materialconstituting the two electrodes and those of possible auxiliary layers(layers for transporting or stopping negative or positive charges) arefixed in such a way that these layers are all transparent ortranslucent.

In this way, an electroluminescent device can be made which emits lighton both its faces.

It is likewise possible, in an inherently-known manner, to form, on theexternal faces of the device according to the invention, one or morefurther auxiliary layers, such as reflective layers, forming a mirror,or such as semitransparent and/or dielectric layers, in order to directthe light emitted by the device or to enhance certain components, inparticular by formation of microcavities.

It is further possible, by superimposing a plurality of devicesaccording to the invention, for example three, each emitting light onboth its faces, these devices comprising layers of differentelectroluminescent organic material having different light emissionwavelengths, to prepare a multicoloured display device, operating bymixing colours controlled by varying voltages applied to the differentlayers of this device.

A second type of multicoloured display can be realised with the aid ofelements formed by the juxtaposition of a plurality of devices accordingto the invention, for example three, these devices comprising layers ofdifferent electroluminescent organic material having different lightemission wavelengths, operating by mixing colours controlled by varyingvoltages applied to the different devices forming each element.

A third type of multicoloured display can be prepared with the aid ofelements formed by the juxtaposition of a plurality of devices accordingto the invention, for example three, these devices comprising furtherauxiliary layers favouring the selection of a narrow wavelength rangewithin the light emission spectrum emitted by the electroluminescentorganic layer(s), operating by mixing colours controlled by varyingvoltages applied to the different devices forming each element.

The invention may be better understood thanks to the detaileddescription, which follows, of non-limiting examples of the realisationof embodiments of the device according to the invention, makingreference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view, in section, of a first embodiment of theGaN-based device;

FIG. 2 is a schematic view, in section, similar to that of FIG. 1, of asecond embodiment of the GaN-based device;

FIG. 3 is a schematic view, in section, of an embodiment of theTiO₂-based device.

FIGS. 4 and 5 are diagrams showing respectively the characteristiccurrent—voltage curve and the characteristic light intensity—voltagecurve of the electroluminescent device illustrated in FIG. 1.

FIGS. 6 and 7 are diagrams showing respectively the characteristiccurrent—voltage curve and the characteristic light intensity—voltagecurve of the device illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Manufacture of a First Embodiment of the Device According to theInvention, Such as is Illustrated in FIG. 1.

On a small square plate 1 of sapphire, with a side of 1 cm and athickness of 0.5 mm, a thin transparent layer 2 of gallium nitride GaN,having a thickness of 10 micrometers is formed. For this purpose,gallium nitride layer 2 is coated on plate 1, serving as the substrate,by means of chemical reaction in the vapour phase between galliumchloride GaCl and ammonia NH₃, at a temperature of 1,050° C., in thepresence of helium used as a carrier gas, the substrate being maintainedat the reaction temperature by means of a susceptor heated byhigh-frequency induction. Heating could equally well be performed bythermal radiation and by using another carrier gas than helium, forexample nitrogen.

Instead of gallium chloride, use may equally well be made of a galliumorgano-metallic compound, such as trimethylgallium or triethylgallium.

The thus obtained gallium nitride layer 2 adheres strongly onto thesurface of substrate 1. It features the characteristic of a type nsemiconductor, resulting in stoichiometric defects, in the absence ofany doping element. The surface impedance value of layer 2 isapproximately 10 Ohms.

There is then formed, on the free surface of gallium nitride layer 2, afilm 3 of poly[2,5-bis(cholestanoxy)-1,4-phenylenevinylene] (polymerdesignated by the initials BCHA-PPV) having a thickness of 0.2micrometer. For this purpose, a drop of a solution of BCHA-PPV in xylene(concentration of this solution 10 g/liter) is dropped onto the surfaceof gallium nitride layer 2 and the layer of solution is distributed overthis surface in such a way as to provide it with a uniform thickness, byturning plate 1 round a vertical axis, keeping the free surface of layer2 pointing upwards in a horizontal plane, at a speed of approximately2,000 revolutions per minute (the so-called “spin-coating” process).Plate 1, thus coated with layer 2 and BCHA-PPV solution, is then heatedfor one hour at a temperature of 100° C. in an oven under reducedpressure in a neutral gas (argon). This treatment induces evaporation ofxylene and formation of a hard BCHA-PPV film 3 which adheres well to thefree surface of gallium nitride layer 2, this film having a thickness of0.2 micrometer.

Finally, the free surface of layer 3 is coated with a thin gold layer 4having a thickness of 0.3 micrometer. To this end, gold layer 4 iscoated by evaporation under vacum in an inherently-known manner, byusing a traditional thermal evaporation device.

To form an electroluminescent device, it is sufficient to connect layers2 and 4, which cover plate 1 and are arranged as illustrated in FIG. 1on each side of the electroluminescent polymer film 3, to the negativeterminal and to positive terminal of an electric voltage source 5.

By applying an electrical potential difference of a few volts betweenlayers 2 and 4 which thus constitute respectively the negative electrodeand the positive electrode of the device, layer 2 emits electrons whichare injected into polymer film 3 and layer 4 emits positive charges,designated in general by the term “holes”, which are injected in theopposite direction into film 3. Charges having opposite sign which arethus injected into film 3 combine with one another and subsequentlydecompose, producing an emission of light, in an inherently-knownmanner. The characteristic current—voltage and light intensity—voltagecurves of the electroluminescent device of FIG. 1 are shown respectivelyin FIGS. 4 and 5.

EXAMPLE 2

A Second Embodiment of the Device According to the Invention isIllustrated in FIG. 2.

This embodiment is in all respects similar to that of FIG. 1 and differssolely in that, on the one hand, a transparent layer 6 of materialfavouring the electron transport (this material consisting of aluminium8-hydroxy-quinoline, a compound commonly designated by the denominationAlq3) and a transparent layer 7 of material constituting a positivecharges-stopping layer (this material consisting of2-(4-biphenyl-5-(tertbutyl-phenyl) 1,3,5-oxadiazole, a compound commonlydesignated by the term “butyl-PBD”) are inserted between gallium nitridelayer 2 and electroluminescent material layer 3 and, on the other hand,in that the holes-emitting electrode is constituted by a transparentlayer 4 a of indium and tin oxide (a commercially available productdesignated by the denomination ITO) having a thickness of 0.15micrometer.

Layers 6 and 7 each have a thickness of 0.02 micrometer.

EXAMPLE 3

Manufacture of a Third Embodiment of the Device According to theInvention, Such as is Illustrated in FIG. 3.

A thin, transparent layer 32 of amorphous titanium oxide TiO₂, stronglydoped with aluminium, is formed on a small square glass plate 1 with aside of 1 cm and a thickness of 1 mm. To this end, a 10 nm thickaluminium layer is first evaporated, then a 10 nm thick TiO₂ layer,follows by a new 1 nm thick aluminium layer, are pulverised with the aidof a magnetron and so on until the total thickness of layer 2 is 50 nm.Once the operation has been completed and after thermal homogenisationtreatment at 300° C. for one hour, under an oxygen atmosphere, it isfound that aluminium has merged with titanium oxide in such a way thatthe final layer of merged TiO₂ is transparent and features an resistanceof the order of 100 Ohms for an element with a square surface.

As in Example 1, a layer 3 of electroluminescent BCHA-PPV polymer isthen formed by spin-coating.

Finally, a thin layer 4 a of ITO, obtained in an inherently-known mannerby pulverisation on a target of ITO by ionic bombardment, is applied onthe free surface of layer 3.

The use of this electroluminescent device is in every respect similar tothat of the device of Example 1. The characteristic current—voltage andlight intensity—voltage curves of the electroluminescent deviceillustrated in FIG. 3 are indicated respectively in FIGS. 6 and 7.

What is claimed is:
 1. An electroluminescent device, comprising a layerof electroluminescent semiconductive organic material, arranged betweena first electrode, constituted by a material having a property ofinjecting electrons into said layer of electroluminescent material, anda second electrode, constituted by a layer of electrically conductivematerial having the property of injecting holes into the layer ofelectroluminescent material, characterized in that said first electrodeis in a form of a transparent or translucent layer of a type nsemiconductor material chosen from among mineral nitrides.
 2. A deviceaccording to claim 1, characterized in that said semiconductor materialis chosen from among gallium nitride GaN, binary alloys of galliumnitride and indium nitride, of general formula Ga_(x)In_((1−x))N, binaryalloys of gallium nitride and aluminium nitride and ternary alloys ofgallium nitride, indium nitride, and aluminium nitride, of a generalformula Ga_(x)Al_(y)In_((1−x−y))N, and mixtures of at least two of thesecompounds and alloys, where x and y each represent a number between 0and 1, a total of x+y being at most equal to
 1. 3. A device according toclaim 2, characterized in that gallium nitride is in asub-stoichiometric state or in a state of being doped by at least oneelement chosen from among groups 4a and 6a of the periodicclassification table.